Holographic optical elements have found many applications in imaging systems, optical wireless communication, data
storage etc. We have developed filter for Lidar receiver which includes holographic optical elements (HOEs) - volume
diffraction grating (VDG) and holographic lens. HOEs were designed and recorded to meet system requirements.
RL Associates Inc. has designed a novel 12-channel, multi-spectral beam splitter which covers the visible
region of the spectrum. Commonly-used beam splitting techniques include dichroic mirrors, spectral
bandpass filters, ruled diffraction gratings, and prisms, all of which are large and difficult to handle. The
RL Associates' beam splitter, developed for the US Navy's multi-spectral imaging programs, uses
multiplexed volume holographic gratings. A volume holographic optical splitter offers multi-channel
splitting in a single element which can be customized to accommodate a broad range of sensor array
geometries. The RL Associates beam splitter has an efficiency of 75% or greater, and an angular
acceptance of up to 25 degrees. These gratings are written in a thermally stable photosensitive glass.
Currently, a 12-channel beam splitter has been developed by multiplexing three volume holograms in a 2x2
array with each element being offset by approximately 35 nm. The output of each element is then directed
toward a separate imaging detector. Each detector is placed at a specific, unique position relative to the
other detectors. This ensures that no cross talk occurs between beam splitting elements. A prototype beam
splitter is currently under development. This beam splitter will extend into the short-wave infrared region
while still working in the visible region.
Measurements and lidar calculations have been made for 1574 nm laser light pulsed through hydrocarbon smoke generated by wood. A pulsed laser signal is directed to the end of wood smoke filled chamber. The signal is reflected back through the smoke by a mirror and the end of the chamber and the total returned energy is measured as a function of the smoke density. The results are compared with a lidar calculation using Rayleigh-Debye-Ganz scattering theory for fractal aggregates. Measurements and calculations are also made of the total backscattered signal for a smoke chamber with a non-reflecting surface. Relatively good agreement between the theory and experimental results are achieved in both cases. These results are used in the feasibility studies of a FireLidar active imaging system being developed for use in search and rescue in smoke and flame environments.
FireLidar, an active optical imaging system, is being developed for use as an aid to search and rescue in smoke and flame environments. The system is intended to augment currently available passive thermal imaging technology by imaging in the presence of a thermal bloom, heavy smoke conditions, or species which strongly absorb thermal radiation, such as water. We present experimental verification of a theoretical model for FireLidar. Lidar range equations for compartment fire scenarios are derived and compared to measurements taken in a controlled smoke chamber. Extinction measurements of near-infrared light through soot particulate provide information about optical properties of fire environments necessary to predict Lidar returns. Measured extinction values are compared to a single-scattering approximation, based on the Rayleigh-Debye-Gans scattering theory for fractal aggregates. Component specifications for a FireLidar prototype system are discussed, including laser power, filter bandwidth, and camera integration times. A man-portable prototype system using specified components is scheduled for completion by the end of 2005, with a handheld device following soon thereafter.